Stretching sprained muscles

Hello, I recently started mobility and stretching following F1 and the stretch courses, but an old injury keeps bugging me. I strained (or sprained) my left QL about 4 years ago for the first time while working as a carpenter carrying 4'x8' plywood panels. Since then I have had problems with tightness in that muscle and knots (or scar tissue). I have also hurt it about once a year from that first injury. Never much (not even the first one) always a few days of rest and I was good to go. The thing is, I've never been able to get range of motion back in it without it "locking up" and feeling tighter after. Sometimes I'm lucky and light stretching (the hurdler mostly) will be ok and help a bit with it. I have strengthened it in the past with deadlifts and various kettlebell carries witch help strengthen the QL, but the tight sensation and knot that I have in it remain. I now work as a landscaper/gardener and it usually bothers me when I work long days kneeling/bending forward.

I have seen 3 physical therapists through the years for correctives and see a chiropractor regularly for maintenance but none of them seem to have been able to help me get rid of the problem. I tried some massages with a tennis ball or rumble roller too.

I just might be missing some crucial part of the equation to help me get that muscle back to normal working order, anyone have insight on how to go about stretching/mobilizing?

Considering I'm not getting much feedback on this topic, I'd like to try Jefferson Curls to see if weighted mobility drills might help. Can anyone tell me how often and what kind of rep scheme I should be using? I'm doing F1 and the stretch series with coach's recommended schedule. Since I'm just starting I'm mostly in the first steps too and I know Jcurls don't come 'till PE2 but I am looking for anything that can help me limber up and strengthen my quadratus lomborum.

My suggestion it is to let the physioterapist to manipulate the scar tissue first then procede with static stretching.

I have my chiropractor who also does ART work on it. But since the QL is such a deep muscle it doesn't seem to get rid of it completely. I got some trigger point therapy massages this past summer too.

This is why I ask about alternatives to try out. But thanks for the answer Alessandro.

Might using heat pads on it before stretching work help?

Benjamin, with the blessing of your physic, heat pads might help for sure. For other alternatives research self myofascial release techniques around your injury.